A lot can happen in 40 million years, the approximate length of the
Cambrian period. Animals showed dramatic diversification during this
period of Earth's history. This has been called the "Cambrian Explosion".
When the fossil record is scrutinized closely, it turns out that the
fastest growth in the number of major new animal groups took place during the
Tommotian
and Atdabanian stages of the Early Cambrian, a period of
time which may have been as short as five million years! In that time,
the first undoubdted fossil
annelids,
arthropods,
brachiopods,
echinoderms,
molluscs,
onychophorans,
poriferans, and
priapulids show up in rocks all over the world.
Perhaps we should call this bit of time the "Tommotian Explosion".

Stratigraphic boundaries are determined by the occurences of fossils.
For instance, the trace fossil Phycodes pedum marks the base
of the Cambrian. This boundary is an unusual case, since stratigraphic
boundaries are normally defined by the
presence or absence of groups of fossils, called assemblages.
In fact, much paleontological work is concerned with questions surrounding
when and where stratigraphic boundaries should be defined.
At first glance, this may not seem like important work, but consider this.
If you wanted to know about the evolution of life on Earth, you would
absolutely have to keep track of time. Questions such as: "how long did
something stay the same?" or,"how fast did it change?" can only be assessed
in the context of time.